Exemplary embodiments of the present invention relate to a semiconductor memory device, and more particularly, to a fuse of a semiconductor memory device.
If even a single defect occurs during fabrication of a semiconductor memory device, the device may not function properly as a memory, and thus become defective. Although the defect occurs only in a cell of a memory, the overall device that includes the defective memory cell (“cell”) may need to be discarded, which results in inefficient yields.
Now, the inefficiency in yields may be resolved by forming some spare cells inside a memory device, and replacing a defective cell with a spare cell to repair the overall memory. A repair operation using spare cells is generally performed by first pre-installing a spare word line prepared to be replaced with a normal word line and/or a spare bit line prepared to be replaced with a normal bit line. Next, a normal word line, or a normal bit line which includes a defective cell, is replaced with the spare word line or the spare bit line. To be specific, when a defective cell is detected by a wafer test, a program for changing an address of the defective cell with an address of a spare cell may be executed inside an internal circuit. Therefore, when the memory with the defective cell is used, and an address signal indicating the defective cell is input, the data of the spare cell (that replaces the defective cell) is accessed.
A widely used address changing method uses a program to change the path of an address by firing a fuse with a laser beam to blow the fired fuse. For example, a typical semiconductor memory device includes a fuse unit for changing an address path by irradiating a fuse with a laser and blowing the fuse. A fuse unit includes a plurality of fuse sets, each of which can replace one address path. The number of fuse sets in a fuse unit depends on the number of spare word lines or spare bit lines that are provided on the margin area of a semiconductor memory device. Each fuse set may include a plurality of fuses for addresses, and an address path is replaced by selectively blowing the fuses for addresses.
A fuse unit may be provided with multiple fuses and a fuse guard ring for protecting an internal circuit from impurities penetrating the internal circuit through a fuse region. However, the metal around a fuse may be corroded due to a voltage applied to the fuse after the fuse is blown.